Over this last year we have identified methylthiopropionate, methylmercaptan and hydrogen sulfide as intermediates of methionine catabolism in the rat liver homogenate system. Studies have been initiated to determine the relative importance of the transaminative and transsulfuration pathways of methionine catabolism using a suicide inhibitor of cystathionase, propargylglycine. A comparison of sulfate and cystathionine accumulation with increasing levels of methionine should allow assessment of the relative importance of these pathways with varying concentrations of methionine. The possibility that methylthiopropionate sulfur may be incorporated into cysteine will be investigated by isolating the cysteine incorporated into metallothionein after rats have been injected with zinc to increase metallothionein synthesis. 35S-methylthiopropionate or 35S-cysteine will be injected six hours after the zinc in these experiments. Additional studies will involve measuring the activity of methionine decarboxylase in liver and other organs. This enzyme is the second enzyme in the transaminative pathway. Preliminary studies have revealed that 20-50 percent of the decarboxylase activity may be cytosolic with the remainder found in the mitochondria. Substrate specificity and kinetic parameters of the cytosolic and mitochondrial enzyme will be assessed.